By ceramic coating, is meant within the meaning of the present invention, a synthesized coating by sol-gel process from a liquid phase precursor based solution, which converts into a solid through a set of chemical reactions (hydrolysis and condensation) at low temperature. The, thus obtained coating can either be organo-mineral or entirely mineral.
By organo-mineral coating is meant within the meaning of the present invention, a coating whereof the network is substantially inorganic, but comprises organic groups, particularly due to the nature of the precursors used and the curing temperature of the coating.
By entirely mineral coating is meant, within the meaning of the present invention, a coating consisting of a completely inorganic material, free from any organic group. Such a coating can also be obtained by sol-gel process with a curing temperature of at least 400° C., or from tetraethoxysilane (TEOS) type precursors with a curing temperature which can be lower than 400° C.
These ceramic coatings are currently witnessing an important development in the field of culinary articles, and particularly by way of coatings covering the inner or outer surface of culinary articles. Furthermore, they have the advantage of making it possible to obtain particularly temperature resistant colored coatings and having easy cleaning properties.
Such coatings are commonly obtained by combining silicon based metal alkoxides such as silanes, or aluminum based ones such as aluminates, and can be applied on metal substrates, such as aluminum, cast aluminum or stainless steel.
However, these sol-gel coatings have limited mechanical resistance properties on metal support, and/or do not permit an optimal thermal conduction, owing to their thermally insulating character. In fact, in the case for example of culinary articles, part of the energy provided to said article is dissipated in the insulating coating instead of being directly transmitted to the food to be cooked. Moreover, with a sol-gel coating which does not conduct or badly conducts heat, a local formation of hot points, unfavorable to the cooking quality, can be observed at the area in contact with the heating member.
Nevertheless, it is essential to be able to have thermally resistant coatings at more than 200° C., that conduct heat in a homogenous and effective manner in their final usage, and which in addition, are mechanically resistant to the usage aggressions (scratches, abrasion . . . ).
In order to resolve such a problem, the skilled person would normally consider the use of standard metal fillers of micrometric size. However, these fillers have a high density, making their suspension difficult and generating a risk of rapid decantation in a formulation containing them.
The applicant has thus, sought to resolve this density problem by incorporating fillers having a low bulk density and a high thermal conductivity. Diamond and cubic boron nitride particles fulfill these two constraints and their use in ceramic coatings obtained by sol-gel process is known by the skilled person.
Thus, U.S. Pat. No. 6,939,908 teaches the use, in transparent ceramic coatings obtained by sol-gel process from a precursor such as an alkoxysilane, of abrasion-resistant particles of aggregate type with a diameter ranging between 0.05 μm and 20 μm for improving the coating deformation resistance, as well as the use of ceramic nanoparticles, of diameter less than 0.1 μm for increasing the transparency of the coating. It consists of transparent coatings for plastic substrates substantially intended for ophthalmic type, motor vehicle glass or airplane cockpit applications. Such coatings can also be applied on metal substrates in order to improve their hardness and corrosion resistant properties.
Furthermore, patent application US 2011/0127461 describes a thermo-conductive composition obtained by sol-gel process, which consists in a polysiloxane matrix wherein inorganic particles are dispersed, such as boron nitride or diamond particles, of dimension ranging between 0.003 and 50 μm, the particles and matrix being chemically bonded. Due to their high thermal conductivity, the diamond or CBN particles favor the transmission of heat inside the rather thermally insulating polysiloxane matrix.
The sol-gel coatings have a thickness ranging between 200 and 500 μm. These consist of very thick coatings intended to be used in the field of electronic power as material for sealing and protecting semi-conductive members. The mechanical resistance properties are not sought for these type of coatings.
In particular, the high content in particles necessary for ensuring the thermal conduction under a thick layer of coating generates a high risk of chipping, which is acceptable for a hardly mechanically solicited electronic application, but is unacceptable for a culinary application that is subjected to constant mechanical aggressions (impacts, abrasion, repeated shocks).
However, the sol-gel coatings taught by these documents do not make it possible to resolve the technical problem of the present invention, which consists in jointly and efficiently improving the mechanical resistance of a sol-gel coating of low thickness (particularly less than 100 nm) and its thermal homogeneity during usage.